In recent years, active matrix operation systems using, for example, a thin-layer transistor, have been actively developed by virtue of their excellent display performance. Among them, a so-called TFT display system is the most representative system, and a further improvement in the display performance has been made. Accordingly, also with respect to requirements for a liquid crystal alignment film, it has now been required to improve various properties at the same time. As the liquid crystal alignment film properties usually required for a so-called TFT display system, the tilt angle of liquid crystal molecules, the voltage holding ratio and the charge-accumulation property by a direct current voltage are, for example, the most basic required properties. An alignment film material satisfying all of these properties has been desired. Namely, a treating agent for liquid crystal alignment has been desired which provides a liquid crystal alignment film presenting a relatively high tilt angle of at least a few degrees and having a high voltage holding ratio and a sufficiently low charge-accumulation by a direct current voltage.
As an example of such a treating agent for liquid crystal alignment, one disclosed in JP-A-8-220541 may be mentioned.
However, in order to develop new liquid crystal display devices of recent years or to improve the performance, a treating agent for alignment has also been required which provides a liquid crystal alignment film presenting a tilt angle of liquid crystal molecules as low as at most 2.degree. and having a high voltage holding ratio and a sufficiently low charge-accumulation property by direct current voltage.
In this connection, a material showing a low tilt angle and a high voltage holding ratio has heretofore been found. However, one having a tilt angle of at most 2.degree. has not necessarily been satisfactory with respect to the very low charge-accumulation property.
Namely, with conventional treating agents for liquid crystal alignment, it has been common to improve the film properties by employing one type of a polyimide or a polyimide precursor and selecting its structure variously. It is known that all these properties are affected by the structure of the polyimide to be used, and in fact, it has not been accomplished to fully satisfy all the properties by one polymer structure as an alignment film. Further, with conventional treating agents for alignment, for example, one can show an excellent property for the voltage holding ratio, but has a large charge accumulation, and another is excellent in the voltage holding ratio and the charge-accumulation property, but has a large tilt angle. Thus, no material has been found which satisfies these three properties adequately at the same time. Further, for the practical mass production of liquid crystal devices, not only the above basic properties but also various properties such as adhesive properties to a substrate, printing properties and rubbing resistance are required.
It has been required to develop various properties including productivity as well as the above-mentioned basic properties, and in fact, adequate performance has not necessarily been obtained only by a single polyimide resin.
Conventional treating agents for liquid crystal alignment are generally, for example, the one obtained by coating a varnish having a solvent-soluble polyimide dissolved in a solvent on a substrate, and baking it to form a polyimide film, and then subjecting it to rubbing treatment to obtain a liquid crystal alignment film, or the one obtained by coating a polyimide precursor solution on a substrate, and baking it usually at a temperature of at least 150.degree. C. for imide-modification to form a polyimide film, and then subjecting it to rubbing treatment to obtain a liquid crystal alignment film.
Here, the liquid crystal alignment film formed by a solvent-soluble polyimide is known to have a problem that even with one having a low tilt angle of at most 2.degree., the thermal stability is inadequate, or even with one having a constant low tilt angle and an excellent voltage holding ratio, the charge-accumulation property is not adequately low. Further, it is known to be usually poor in the adhesive properties to a substrate or in the printing properties.
On the other hand, the one employing a polyimide precursor has advantages that a high and more stable tilt angle can be obtained and the charge-accumulation property can be made small, or advantages that the adhesive properties to a substrate and the printing properties are excellent. However, on the other hand, it has had drawbacks that it is difficult to obtain an adequately low tilt angle, the voltage holding ratio is poor in many cases, the properties tend to depend on the baking temperature, and if the imide-modification degree is not sufficient, the solvent resistance of the film surface is poor.
Namely, both the solvent-soluble polyimide and the polyimide precursor have advantages and disadvantages contrary to each other as a liquid crystal alignment film. Accordingly, it is not necessarily easy to satisfy all the properties required for an alignment film for a so-called TFT.
As mentioned above, in order to improve the performance of new liquid display devices or conventional devices, an excellent treating agent for liquid crystal alignment has been desired which provides a constant low tilt angle of at most 2.degree. and which satisfies properties such as a high voltage holding ratio and a very low charge-accumulation property, at the same time.
Namely, the object of the present invention is to present a treating agent for liquid crystal alignment excellent in these properties.